All glow-in-the-dark products contain phosphors. A phosphor is a substance that radiates visible light after being energized. The two places where we most commonly see phosphors are in a TV screen or computer monitor and in fluorescent lights. In a TV screen, an electron beam strikes the phosphor to energize it (see How Television Works for details). In a fluorescent light, ultraviolet light energizes the phosphor. In both cases, what we see is visible light. A color TV screen actually contains thousands of tiny phosphor picture elements that emit three different colors (red, green and blue). In the case of a fluorescent light, there is normally a mixture of phosphors that together create light that looks white to us.
Take a phosphor that fits the bill, mix it in with the plastic to be molded into the product, and you have yourself a glow-in-the-dark whatever. Light from the sun or the living room lamp energizes the phosphors in the plastic and excites them, and with the lights off, you can watch as their atoms slowly lose this extra energy in the form of a dim glow.
Scientists have created numerous phosphors in the lab, but zinc sulfide and strontium aluminate are the ones that are most commonly used in glow-in-the-dark products, with strontium aluminate being the longer lasting of the two. The chemicals are mixed right in with the plastic that is molded into glow in the dark stars for your ceiling or added to the pigment of your Halloween make-up.
On rare occasions, something will glow in the dark without needing to be charged. These items still use phosphors to create the glow, but they add a radioactive element like radium to the compound. The radioactive element gives off small amounts of radiation, not enough to be dangerous, that constantly charge the phosphors in the same way a light would. Radiation-charged phosphors are typically used on clock or watch hands that need to glow hours after a light has been turned off.
Another way to make objects glow in the dark is through chemiluminescence, a chemical reaction. Two chemicals are mixed together, and the resulting reaction causes electrons to become excited, moving to a higher energy level. When the electrons return to normal levels, they release light energy, producing a glow. This is the type of reaction that is used to create the light in glow sticks.